For some time, ion engines have been used for propulsion of vehicles in space. Ion engines use movement of ions to provide thrust. Outside of space propulsion, ion generation and acceleration may also be applied to various types of materials processing systems involving ion sources, such as ion beam etching or micro-machining.
Generally, an ion thruster utilizes three separate and independent processes to generate thrust; these include ion generation within a discharge chamber, ion acceleration within an ion-optics assembly, and neutralization of the positive ion beam by electrons produced in a neutralizer cathode. An ion engine works by generating plasma ions within a discharge chamber via an electron bombardment process. The ions leave the thruster through the ion-optics assembly consisting of charged screen, accelerator and decelerator grids. The net force from the ions leaving the thruster housing generates a thrust. A neutralizer is located outside the thruster housing and generates electrons that current—and charge—neutralize the ion beam.
In normal operation, the ion-optics assembly serves two main purposes, consisting of ion acceleration (as mentioned above) and as a means of preventing electrons present within the beam plasma from being accelerated into the discharge chamber. The ion optics assembly is the key element of an ion thruster that enables separation of the ion generation, acceleration, and neutralization processes. The key element of the ion optics assembly that causes the separation of these processes is the negatively biased accelerator grid. In general, the magnitude of the accelerator grid potential directly determines whether electrons from the beam plasma will backstream into the discharge chamber. It is very common in the ground-based testing of ion engines to measure the minimum |accel voltage| where an accel grid can no longer retard electrons present in the beam plasma from backstreaming through the ion optics assembly and into the discharge chamber of an ion thruster. The onset of backstreaming is detected by decreasing the voltage of the accel supply (from an initial voltage of ˜|−500| V) and monitoring the ion beam current. A large increase in the beam current occurs when the accel voltage is decreased below a value commonly termed the “backstreaming limit.” The backstreaming limit is generally determined by the accel grid geometry. The region of the accel voltage/beam current curve near the backstreaming limit is non-linear. This is because the operational principle of the ion thruster (i.e., the separation of ion production, acceleration and neutralization processes) breaks down as energetic backstreaming electrons begin to interact within the ion optics assembly and discharge chamber.
To prevent backstreaming, typical ion engines use a fixed accel voltage that has a sufficient magnitude to ensure some margin of safety. Unfortunately, as the accel grid erodes over a period of time which causes its geometry to change, the backstreaming limit changes (i.e., increases). Therefore, the optimal accel voltage to prevent backstreaming also changes over the lifetime of the ion engine. Using a large magnitude fixed accel voltage causes the accel grid to wear out faster near the beginning and middle of a mission and fail sooner than would be desirable. Once the backstreaming limit is reached, a typical ion engine would suddenly (over ˜10 to 100 hr) stop producing thrust efficiently. To circumvent this problem, current ion propulsion systems could increase their lifetimes by selecting an even higher accel voltage, but this would cause more accel erosion and pose a larger contamination threat to the spacecraft.
The disadvantages associated with the conventional fixed accel voltage selection techniques for an ion engine have made it apparent that a new technique for accel voltage selection is needed. The new technique should increase the useful life of the ion engine and should not pose a larger contamination threat to the spacecraft. Also, it would be desirable if the new technique could estimate the remaining lifetime of the ion thruster. The present invention is directed to these ends.